1. Field of the Invention
The present invention generally relates to an optoelectronic sensor for detecting moisture on a window and, more particularly, to such an optoelectronic sensor having a prism structure which couples radiation from a transmitter into the window and decouples reflected radiation out of the window towards a receiver in which the prism structure includes individual prisms having prism surfaces in coupling areas arranged perpendicular to the input radiation angle of incidence and prism surfaces in decoupling areas arranged perpendicular to the output radiation angle of incidence.
2. Background Art
DE 197 20 874 C2 describes an optoelectronic sensor based on the disturbed total reflection of light principle. A first optical system (i.e., a first lens) parallelizes light from a transmitter and directs the light towards a coupler at a 45° input angle. The coupler couples the light into the internal side of the window at the 45° input angle. If the window is dry and clean, then light reaching the external side of the window is totally reflected back towards the coupler at a 45° output angle. The coupler decouples the reflected light (i.e., the useful light) towards a second optical system (i.e., a second lens) at the 45° output angle. The second lens directs the reflected light towards a receiver. Wetting of the window decouples the light from this optical path such that total reflection no longer takes place.
The coupler is a prism structure having a plurality of individual prisms. The prisms have prism surfaces in coupling areas arranged perpendicular to the 45° input angle and prism surfaces in decoupling areas arranged perpendicular to the 45° output angle. Each prism includes a side flank in addition to a prism surface. The flanks of the prisms extend perpendicular to the prism surfaces. As such, the prism flanks in the coupling areas are parallel with light transmitted into the window at the 45° input angle and the prism flanks in the decoupling areas are parallel with the useful light reflected from the window at the 45° output angle.
A problem is extraneous light influences cannot be completely excluded. Extraneous light reduces the sensitivity of such a sensor and may cause the sensor to trigger incorrectly. As such, a reduction in extraneous light results in a functional improvement of such a sensor.
Theoretical and experimental considerations identify an optical path on which extraneous light from the external environment can impinge on the receiver through the coupler and the second lens. Extraneous light incident at certain angles near the angle perpendicular to the window can impinge on the prism flanks and emerge glancing off these flanks. The second lens, which focuses the useful light reflected off the window towards the receiver, has an angular acceptance that is not insignificant; and the receiver includes an extensive surface. As a result, this extraneous light can reach the receiver.